1. Field of the Invention
This invention relates to a process for coating a diamond-like carbon film on the surface of various materials and a thin strip coated by this process.
2. Description of the Related Art
A diamond-like carbon film can be formed according to the CVD (Chemical Vapor Deposition) gas phase growth methods such as disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 61-272363, 62-67174, 63-123802, 1-167211, and 2-196095. According to these processes, although a coating with an adhesive force strong enough to stand practical application for limited materials such as tungsten carbide, etc., is possible, it is difficult to coat a diamond-like carbon film onto various materials such as iron type materials, ceramics, and plastics with an adhesive force strong enough to with stand a practical application.
Accordingly, to improve this adhesive force of diamond-like carbon film, techniques have been developed in which an element which can readily form a carbide is deposited as the intermediate layer on the surface of a substrate made of a material with a weak adhesion to a diamond-like carbon film (Japanese Unexamined Patent Publications (Kokai) Nos. 61-163273, 64-62213, 64-62457, 64-65092, 64-79371, 1-147067). Nevertheless, although the adhesion to diamond-like carbon film could be improved by such methods, due to increased material and production costs, they have not been adopted for the industrial production of materials composed primarily of iron.
Prior arts have enabled a thin film having a thickness of 100 .mu.m or less from the thickness of one atom layer to be formed and utilized in the semiconductor industries and semiconductor researches. In the field of semiconductor materials, however, it is necessary to remove defects on the atomic order (presence of unintended impurities, distortion on the atomic order), which have an adverse influence on the motion of electrons in the material, as much as possible, and consequently, for the formation of a thin film in the semiconductor field, there is used a dry process which can effect a film formation with gas molecules or a plasma excited species having as small as possible kinetic energy, i.e., some 10 eV or less. Such a technical concept has been erroneously applied to the forming of a diamond-like carbon film, and therefore, most researchers have practiced the formation of diamond-like carbon films according to the CVD method, and thus ultimately were able only to obtain a diamond-like carbon film with a weaker adhesive force, compared with one obtained by the PVD method.
The techniques for forming a diamond-like carbon film by the PVD (Physical Vapor Deposition) method are disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 59-174508, 60-145994, 1-097839, 2-274876. In these methods, for substrates composed mainly of materials which can readily form carbides of, for example, silicon, tungsten, and chromium, etc., diamond-like carbon films can be coated with stronger adhesive force than by the CVD method, but for materials composed mainly of iron, the coating still has only a weak adhesive force. For example, if a diamond-like carbon film is directly formed on pure iron by using a Kaufman type ion source, when the sample film after formation is taken out of the vacuum chamber and into the air, it has an adhesive force only to an extent such that a part of the film is easily peeled off due to the pressure of moisture in the air, and when the portion not peeled off is scratched with the tip of a pincette, that portion is also easily peeled off. Therefore the adhesive force obtained can not withstand industrial use. To solve this problem, there have been developed a method of ion injecting carbon atoms onto from the substrate surface of an iron type material, or causing a penetration of carbon atoms by a method such as thermal diffusion, and coating a diamond-like carbon film thereon by the PVD method (Japanese Unexamined Patent Publications (Kokai) Nos. 1-162757 and 2-122075). Such methods, however, did not give a sufficient improvement of the adhesion of a diamond-like carbon film to an iron type substrate.